Light olefins serve as the building blocks for the production of numerous chemicals. Light olefins have traditionally been produced through the process of steam or catalytic cracking. Propylene, a light olefin consisting of three carbon atoms wherein two of the carbon atoms are joined by a double bond, has a great number of commercial applications, particularly in the manufacture of polypropylene, isopropyl alcohol, propylene oxide, cumene, synthetic glycerol, isoprene, and oxo alcohols. When propylene is produced in the presence of hydrogen, it is often accompanied by the formation of propane. Propane is a paraffin, a saturated hydrocarbon which is used as a component of household fuel, as an extractant, a refrigerant, or an aerosol propellant. Generally, it is required to separate propane from propylene before the propylene can be used to produce more valuable products. However, the boiling points of propane and propylene are very close to one another and separating propane from propylene has traditionally required an energy-intensive fractionation process known as superfractionation. Superfractionation generally refers to fractional distillation of compounds having the same number of carbon atoms per molecule.
The reaction product produced by a conversion process such as fluid catalytic cracking, pyrolysis of naphtha, and conversion of methanol to olefins is a light gas stream containing lighter components (e.g. hydrogen, nitrogen, etc.) methane, ethane and a substantial quantity of hydrocarbons of higher molecular weight, for example, propane, butane, pentane, and often their unsaturated analogs. Separation of these components to recover propylene requires a complex energy-intensive scheme, thus creating a need for more efficient separation processes which yield higher recovery levels of propylene. A typical ethylene separation section of an ethylene plant containing both cryogenic and fractionation steps to recover an ethylene product with a purity exceeding 99.5% ethylene is described in an article by V. Kaiser and M. Picciotti entitled, "Better Ethylene Separation Unit," that appeared in Hydrocarbon Processing, November 1988, pages 57-61 and is herein incorporated by reference. In the production of propylene,: a by-product of the conversion process, a reactor effluent stream recovered from the conversion process is passed to a complex series of separation stages involving a combination of compression and fractionation steps to recover a C.sub.3 hydrocarbon stream. Conventionally, the C.sub.3 hydrocarbon stream is passed to a superfractionator known as a C.sub.3 splitter to perform the separation of propylene from a feedstream consisting essentially of propylene and propane to produce a high purity propylene stream and a propane-containing stream. The C.sub.3 splitter, or superfractionator, makes the final separation between propylene and propane. In this separation, propylene is removed as an overhead stream, and the propane stream is removed as the C.sub.3 splitter bottom stream. The higher the purity of the propylene desired, the more energy intensive is the degree of superfractionation in the C.sub.3 splitter. Typically, the C.sub.3 splitter or propane/propylene splitter requires so many theoretical separation stages, or trays, that the column is constructed as two separate towers: a rectifier and a stripper. Lighter components such as hydrogen and methane which may be present in the C.sub.3 hydrocarbon stream further complicate the separation.
Pressure swing adsorption (PSA) provides an efficient and economical means for separating a multi-component gas stream containing at least two gases having different adsorption characteristics. The more strongly adsorbable gas can be an impurity which is removed from the less strongly adsorbable gas which is taken off as product; or, the more strongly adsorbable gas can be the desired product, which is separated from the less strongly adsorbable gas. In PSA, a multi-component gas is typically fed to at least one of a plurality of adsorption zones at an elevated pressure effective to adsorb at least one component, while at least one other component passes through. At a defined time, the feedstream to the adsorber is terminated and the adsorption zone is depressurized by one or more co-current depressurization steps wherein pressure is reduced to a defined level which permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. Then, the adsorption zone is depressurized by a counter-current depressurization step wherein the pressure on the adsorption zone is further reduced by withdrawing desorbed gas counter-currently to the direction of the feedstream. Finally, the adsorption zone is purged and repressurized. The combined gas stream produced during the counter-current depressurization step and the purge step is typically referred to as the tail gas stream. The final stage of repressurization is typically performed by introducing a slipstream of product gas comprising the lightest gas component produced during the adsorption step. This final stage of repressurization is often referred to as product repressurization. In multi-zone systems, there are typically additional steps, and those noted above may be done in stages. U.S. Pat. No. 3,176,444 issued to Kiyonaga, U.S. Pat. No. 3,986,849 issued to Fuderer et al., and U.S. Pat. Nos. 3,430,418 and 3,703,068 both issued to Wagner, among others, describe multi-zone, adiabatic PSA systems employing both co-current and counter-current depressurization, and the disclosures of these patents are incorporated by reference in their entireties.
U.S. Pat. No. 5,672,197, hereby incorporated by reference, discloses a process for the separation of a mixture of gases wherein an internal gas flow is pumped througha plurality of beds containing a pressure dependent adsorbent. The beds are connected in series. Gas is pumped from the most upstream of the beds to generate the internal gas flow, displacing gas through the series in the downstream direction. A first inlet flow is withdrawn from the most upstream bed. The first outlet flow consists primarily of the most highly adsorbed component. Feedstock is added downstream of the first outlet, separated by at least one bed and a second outlet flow is withdrawn from a second outlet in fluid communication with the most downstream bed in series. At the end of the cycle, gas pressure in the most downstream bed is increased to operating pressure, while pressure in the most upstream bed is reduced to the lower pressure. At the end of the cycle, valves are used to switch positions of the inlet and outlet ports and bed connections so that each bed effectively advances upstream with respect to the internal gas flow by one position, except for the most upstream bed which is effectively moved to the most downstream position.
Various classes of adsorbents are known to be suitable for use in PSA systems, the selection of which is dependent upon the feedstream components and other factors. Molecular sieves such as the microporous crystalline zeolite and non-zeolitic catalysts, particularly aluminophosphates (AlPO) and silicoaluminophosphates (SAPO), are known to promote reactions such as the conversion of oxygenates to hydrocarbon mixtures. Numerous patents describe this process for various types of these catalysts: U.S. Pat. No. 4,310,440 (Wilson et al.), U.S. Pat. No. 4,567,029 (Wilson et al.), U.S. Pat. No. 5,095,163 (Barger), U.S. Pat. No. 5,191,141 (Barger), U.S. Pat. No. 5,126,308 (Barger), U.S. Pat. No. 4,973,792 (Lewis), and U.S. Pat. No. 4,861,938 (Lewis). The above U.S. patents are hereby incorporated by reference.
U.S. Pat. No. 5,744,687 and U.S. Pat. No. 5,365,011 disclose a process for the integration of a PSA zone containing an adsorbent selective for the adsorption of ethylene and propylene from a catalytic cracking process at an adsorption temperature above 50.degree. C. to about 250.degree. C. The adsorbent is selected from the group consisting of zeolite 4A, zeolite 5A, zeolite 13X, and mixtures thereof. The adsorbed ethylene and propylene is desorbed from the adsorbent by reducing the pressure or raising the temperature, or by reducing the pressure and raising the temperature.
A paper entitled, "Olefin-Paraffin Separations by Adsorption: Equilibrium Separation by .pi.-Complexation vs. Kinetic Separation", by S. U. Rege, J. Padin, and R. T. Yang and published in the AIChE Journal, volume 44, 1998, at page 799 and herein incorporated by reference, compares the performance of a PSA process using 4A zeolite, carbon molecular sieve, and a sorbent based on a silica substrate over which silver nitrate has been dispersed for the separation of propylene from mixtures of propylene and propane.
Recovering propylene from cracking, oxygenate conversion, and propane dehydrogenation processes is an expensive and complex process involving extensive compression and superfractionation to separate ethylene from the C.sub.3 hydrocarbons, and finally superfractionation to separate the propylene from the propane. Prior attempts to employ adsorptive separation have found it difficult to achieve both high purity and high recovery of propylene. Processes are sought which enable the concentration and recovery of the propylene from cracking and oxygenate conversion effluent without expensive superfractionation steps.
It is an objective of the present invention to provide a separation process for the production of high purity propylene which does not require superfractionation.
It is an objective of the present invention to provide a process for the production of high purity propylene at a high propylene recovery.